1. Field of the Invention
The present invention relates to a thin film semiconductor having a region which can be regarded as substantially a single crystal (hereinafter referred to as "a monodomain region"), and is formed on a substrate having an insulating surface, and to a semiconductor device using the thin film semiconductor as an active layer. More particularly, the present invention relates to a thin film transistor having an active layer constituted by a crystalline silicon film.
2. Description of the Related Art
In recent years, a technique for constituting a thin film transistor (TFT) by using a thin film silicon film (having a thickness of from several hundred to several thousand of angstroms (.ANG.)) formed on a substrate having an insulating surface has drawn a considerable attention. A thin film transistor is widely applied to electronic devices such as ICs and liquid crystal display devices.
The most important portion of a thin film transistor is, so to speak the heart of thin film transistor, a channel forming region and a junction portion to join the channel forming region with the source/drain region. That is, the active layer has the greatest influence on the performance of a thin film transistor.
As a thin film semiconductor constituting the active layer of the thin film transistor, generally employed is an amorphous silicon film formed by plasma CVD or low pressure thermal CVD.
A thin film transistor using an amorphous silicon film is practically available at present, however, in case higher speed operation is required, a thin film transistor based on thin film silicon having good crystallinity (hereinafter referred to as "crystalline silicon film") must be employed.
For instance, in an active matrix type liquid crystal display device or in a passive type liquid crystal display device, a drive circuit for driving pixel TFTs provided to the pixel region, a circuit for handling image signals, and a memory circuit for recording various types of information, are necessary for the peripheral circuits.
Moreover, in the circuits above, the circuit for processing and controlling the image signals and the memory circuits for recording various types of information are required to have a well performance comparable to that of a known integrated circuit using a single crystal wafer. Accordingly, in case that the above-mentioned circuits are to be integrated by using a thin film semiconductor formed on the substrate, a crystalline silicon film having high crystallinity comparable to that of a single crystal must be formed on the substrate.
As methods for forming a crystalline silicon film on a substrate, known are the techniques disclosed in Japanese Laid-Open Applications 6-232059 and 6-244103 filed by the present inventors. The technique disclosed in the references above comprises forming a crystalline silicon film having excellent crystallinity by utilizing a metallic element capable of accelerating the crystallization of silicon in case of applying heat treatment at 550.degree. C. for about 4 hours.
However, even if the technique mentioned above is employed in the active layer of a thin film transistor, the resulting thin film transistor is still unsatisfactory when used as a transistor constituting various types of arithmetic circuit, memory circuit, or the like, because the crystallinity thereof as an active layer is yet insufficient to suffice the required characteristics.
In particular, it is required that the crystalline silicon film having a crystallinity comparable to that of a single crystal is substantially free of crystal grain boundaries. This is because the grain boundaries function as an energy barrier which obstructs the path of electrons which pass to and fro between the crystals.
The present inventors have classified the crystal growth process into four steps. i.e., from a first to fourth step described below when the above technique is employed. The explanation is given with reference to FIGS. 3(A) to 3(F).
Referring to FIG. 3(A), a silicon oxide film 301 is formed as a buffer layer on the surface of a substrate. An amorphous silicon film 303 is formed thereon. The concave or convex portion 302 (only the convex portion is shown in the figure) is formed by the surface roughness or a dust that was present on the surface of the silicon oxide film.
A solution containing a metallic element that accelerates the crystallization is provided dropwise to the surface of the amorphous film 303, and is applied by spin coating. Thus is obtained a state as is shown in FIG. 3(A), in which a nickel layer 304 is retained on the surface of the amorphous silicon film 303.
The amorphous silicon film 303 is crystallized thereafter by applying heat treatment in the temperature range of from 500 to 700.degree. C. However, in case a glass substrate is used, the heat treatment is preferably effected at a temperature of 650.degree. C. or lower by taking the heat resistance of the glass substrate into consideration.
Then, as is indicated by an arrow in FIG. 3(B), the metallic element undergoes an isotropic internal diffusion inside the amorphous silicon film 303 as to reach the interface with the silicon oxide film 301. This is a first step.
The metallic element then segregates in the concave or convex portion 302 after migration inside the interface between the silicon oxide film 301 and the amorphous silicon film 303. This is a second step. This occurs because the metallic element seeks for an energetically stable site, and hence, in this case, the concave or convex portion 302 provides the segregation site (FIG. 3(C)).
Thus, crystal nuclei generates in the concave or convex portion 302 which functions as the segregation site, because the metallic element is present at a high concentration. In case that the metallic element is nickel, according to the study of the present inventors, crystal nuclei generates when the concentration of nickel is 1.times.10.sup.20 atoms/cm.sup.3 or higher.
Crystal growth initiates from the crystal nuclei. At first, crystallization proceeds in the direction approximately vertical to the surface of the silicon film. This is a third step (FIG. 3(D)).
In the region 305 where crystallization proceeded in a direction approximately vertical to the surface of the silicon film (hereinafter referred to as "the vertical growth region"), the crystallization proceeds in such a manner that the metallic element concentrated at a high concentration is pushed up with respect to the surface of the silicon film. Accordingly, metallic elements are also condensed on the surface of the amorphous silicon film 303 located at the upper side of the concave or convex portion 302. As a result, the vertical growth region 305 is obtained as a region containing the metallic element at a high concentration as compared with the other regions.
Then, crystal growth occurs from the interface 306 of the amorphous silicon film 303 that is in contact with the vertical growth region 305 in a direction approximately in parallel with the substrate (the direction shown with an arrow in FIG. 3(E)). This is a fourth step. The crystals 307 are columnar or needle-like crystals approximately equivalent to the film thickness of the amorphous silicon film 303(FIG. 3(E)).
Because the crystals 307 grow in a direction approximately in parallel with the substrate, the growth stop by colliding with other crystals faced opposed thereto. Thus, the boundary where collision occurs as is shown in FIG. 3(F) becomes the crystal grain boundary 308. The crystal region 309 thus formed becomes a region (hereinafter referred to as "a lateral growth region") having a relatively uniform crystallinity.
Thus, in the conventional crystallization morphology, since numerous segregation sites were formed irregularly in this manner, the crystal nuclei has a high density and each crystal grain obstructs the growth each other. As a result, diameter of the crystal grains becomes small.
That is, in case of forming the active layer of a thin film transistor using a crystalline silicon film formed by the technique described above, for instance, crystal grain boundary is inevitably incorporated inside the silicon film. Accordingly, it is practically impossible to realize a crystallinity equivalent to that of a single crystal.
The diameter of crystal grain can be increased by decreasing the generation density of the crystal nuclei, however, the location of crystal nuclei depends on the segregation site of the metallic element. According to the conventional technique, the sites (for instance, the concave or convex portion 302 shown in FIG. 3(A)) which become the segregation sites are formed irregularly, and it is impossible to control the position thereof.